Maintainer's Corner

Readme for stratosphere-0.24.4

Stratosphere: AWS CloudFormation in Haskell

AWS CloudFormation is a system that provisions and updates Amazon Web Services
(AWS) resources based on declarative templates. Common criticisms of
CloudFormation include the use of JSON as the template language and limited
error-checking, often only available in the form of run-time errors and stack
rollbacks. By wrapping templates in Haskell, we are able to easily construct
them and help ensure correctness.

The goals of stratosphere are to:

Build a Haskell EDSL to specify CloudFormation templates. Since it is
embedded in Haskell, it is type-checked and generally much easier to work
with than raw JSON.

Have a simple checking/linting system outside of the types that can find
common errors in templates.

Be able to also read valid CloudFormation JSON templates so they can be
type-checked. This also gives us free integration tests by using the huge
amount of example templates available in the AWS docs.

Example

Here is an example of a Template that creates an EC2 instance, along with the
JSON output:

Value Types

CloudFormation resource parameters can be literals (strings, integers, etc),
references to another resource or a Parameter, or the result of some function
call. We encapsulate all of these possibilities in the Val a type.

We recommend using the OverloadedStrings extension to reduce the number of
Literals you have to use.

Lenses

Almost every CloudFormation resource has a handful of required arguments, and
many more optional arguments. Each resource is represented as a record type
with optional arguments wrapped in Maybe. Each resource also comes with a
constructor that accepts required resource parameters as arguments. This allows
the user to succinctly specify the resource parameters they actually use
without adding too much noise to their code.

To specify optional arguments, we recommend using the lens operators & and
?~. In the example above, the ec2Instance function takes the AMI as an
argument, since it is required by the EC2Instance resource type. Then, the
optional EC2 key name is specified using the & and ?~ lens operators.

This approach is very similar to the approach taken by the amazonka library.
See this
blog post
for an explanation.

Auto-generation

All of the resources and resource properties are auto-generated from
a
JSON schema file and
are placed in library-gen/. The gen/ directory contains the auto-generator
code and the JSON model file. We include the library-gen/ directory in git so
the build process is simplified. To build library-gen from scratch and then
build all of stratosphere, just run the very short build.sh script. You can
pass stack args to the script too, so run ./build.sh --fast to build the
library without optimization. This is useful for development.

In the future, it would be great to not have to include the auto-generated code
in git.

Contributing

Feel free to raise any issues, or even just make suggestions, by filing a
Github issue.

Future Work

Implement basic checker for things like undefined Refs and duplicate field
names. This stuff would be too unwieldy to do in types, and performing a
checking pass over a template should be pretty straightforward.

Use a custom JSON encoder so the templates look a little more idiomatic. We
also create a lot of empty whitespace and newlines using aeson-pretty. There
are limits on the size of CloudFormation templates, and we want readable
output without hitting the limits. Also, we have some newtypes that just
exist to override aeson instances, and we could get rid of those.

Use a custom JSON decoder with useful error messages. Although we don't use
them, we have implemented FromJSON instances for everything. Theoretically,
stratosphere could be used as a checker/linter for existing JSON
CloudFormation templates.